1. Field of the Invention
The present invention relates to the field of integrated optic circuits or IOCs and more specifically to miniature optical components formed by the mounting of a chip comprising optic waveguides on an optic fiber connection support.
Recent developments in integrated optic circuits enable the incorporation of numerous active or passive optic elements (gratings, mirrors, etc.) and the association of various photonic functions (emitter, detector, diode, laser and other function) or electro-optical functions (modulator, coupler and other functions) in a miniature chip which in this case is practically an optical mini-bench.
In general, an integrated optic circuit is formed by a chip comprising a set of functional optical elements connected by optic waveguides conducting the processed light signals. The optical elements and the waveguides are implanted on the substrate of the chip.
2. Description of the Prior Art
Various types of substrate and different implant technologies have been implemented, in the prior art, to develop integrated optic circuits. In particular, there are known integrated optic circuit structures that associate the following substrates and layout technologies:
silicon (Si) with nitride or silica deposition, PA1 germanium (Ge), PA1 gallium arsenide (GaAs), with epitaxy, PA1 indium phosphide (InP), with epitaxy, PA1 lithium niobate (LiNbO.sub.3), with metal diffusion, PA1 glass, with ion exchange technology, and PA1 polymers, with insulation or molding of guides; and this list is not exhaustive. PA1 a motherboard of anisotropic crystalline matter, having grooves on the surface that are parallel to an axial direction, each groove being adapted to receive an optic fiber, align it in the axial direction and position it in a transversal plane, and PA1 a substrate chip, one face of which comprises optical waveguides, the mounting consisting in carrying the face of the chip against the surface of the motherboard with the special feature wherein: PA1 the surface of the motherboard is hollowed out with female microstructures in the shape of axial buttonholes with sub-millimetrical dimensions, wherein, PA1 the face of the chip has projecting male microstructures formed by metal deposition, capable of fitting into and sliding axially in the female microstructures during the mounting of the component. PA1 at least one plate of anisotropic crystalline material etched on the surface with grooves parallel to the axial direction, each groove being adapted to receive an optic fiber and hold it fixed in the transversal position, the surface of the plate comprising male microstructures formed by metal deposition, capable of fitting into and sliding axially in female microstructures of the motherboard, during the mounting of the component. Preferably, the male microstructures of the plate are mushroom-shaped. PA1 a motherboard of anisotropic crystalline matter having grooves on the surface that are parallel to an axial direction, each groove receiving an optic fiber and aligning it in the axial direction and positioning it in a transversal plane, and PA1 a substrate chip, one face of which comprises waveguides, the mounting consisting in attaching the face of the chip against the surface of the motherboard, the method comprising steps consisting in: PA1 hollowing out female microstructures, in the shape of axial buttonholes, in the surface of the motherboard made of anisotropic crystalline material, the female microstructures having sub-millimeter dimensions, PA1 making a metal deposit to form projecting male microstructures on the face of the substrate chip, and PA1 fixedly joining the chip with the motherboard by fitting the male microstructures into the female microstructures, and by obtaining a sliding motion in the axial direction. PA1 metallizing the face of the substrate chip. PA1 etch the metallization by keeping the metallized zones solely at the locations of the male microstructures. PA1 making an anisotropic etching in the motherboard made of crystalline material. PA1 performing a laser etching of the motherboard. PA1 reducing the thickness of the motherboard at the locations of the female microstructures. PA1 molding the motherboard on a matrix whose surface comprises raised ribs, parallel to an axial direction, and projecting microstructures in the form of axial tongues with sub-millimeter dimensions.
The substrate of an integrated optic circuit may be a semiconductor substance, especially silicon, or a non-conductive substance. The electrical properties are not of vital importance.
The use of integrated optic circuits is of considerable importance in fiber-optic telecommunications networks. In particular, it is being planned to incorporate signal-processing functions, by means of integrated optic circuits, in the communication lines themselves. The integrated optic circuits are thus destined to carry out on-line processing functions such as routing, multiplexing, demultiplexing, modulation, switching, and optic signal amplification.
The main obstacle to the development of the use of integrated optic circuits is the problem of the connection between optic fibers and optic circuit waveguides, because the fibers and guides have microscopic cores and need to be perfectly aligned for the effective transmission of the optical signals.
The problem, firstly, is to obtain very precise positioning of the cores of the optic fibers in the alignment of the axes of the waveguides implanted in a substrate chip.
Secondly, the problem is to obtain a stable, reliable and permanent attachment of the fibers to the waveguides.
In the manual method, each microscopic fiber is made to coincide with the respective microscopic waveguide and then the fiber and the guide are glued together by laboratory methods of mechanical or piezoelectric micropositioning. These methods are costly and cannot be envisaged for industrial-scale production.
There are hybridization methods which start with the positioning of a layer of optic fibers in a series of parallel groves hollowed out in a support so as to pre-arrange the fibers in a plane with a specified spacing between each fiber and the other fibers. Then, an integrated optic circuit is implanted in the substrate of a chip with optic waveguides traced out on the surface of the substrate block. The waveguides extend axially to the edge of the chip so as to be in a plane with the same spacing between them as between the fibers.
The problem then lies in adjusting the chip with respect to the fiber support so as to position the series of waveguides very precisely in the alignment of the layer of optic fibers.
The usual method in which the chip is positioned after the support, the waveguides are aligned in the axis of the fibers by manual action on an optical detection bench and then the chip is glued or soldered to the support has drawbacks of cost, lack of reliability and lack of industrial reproducibility.
An article by E.J. MURPHY, "Fiber Attachment for Guided Wave Devices" in the Journal of Lightwave Technology, June 1998, Vol. 6, No. 6 describes a developed technique that proposes the forming of two microscopic ribs to align a fiber support on the surface of the chip. This support comprises two plates that sandwich the fibers in the grooves. One of the plates is extended so that the plate and the chip overlap partially. The two micrometer-sized ribs of the chip fit into two grooves, free of fibers, hollowed out in supporting plates. The ribs are formed at the same time as the waveguides of the silicon chip, by the etching of the silicon passivation layer on the surface of the substrate.
One drawback of this technique is that the chip and the support are not fixed together but simply laid and aligned on top of each other. There is no mechanical holding whatsoever between the chip and the fiber support. Furthermore, the two micrometer-sized ribs do not prevent a tilting of the support around the axis of the ribs and a loss of alignment.
These methods and this technique are but partial and unsatisfactory mechanical approaches, so much so that they have been supplanted by what are called "flip-chip" techniques.
In the known flip-chip techniques, the chip comprising the waveguides is superimposed on a motherboard hollowed out with axial grooves in which fibers are positioned. The motherboard is therefore used as a support for the fibers and the chip.
The motherboard is generally formed by silicon, a crystalline material in which grooves can be made with excellent directivity. This enables the perfect alignment of the optic fibers and their precise positioning in a transversal plane.
The fibers are held fixed in the grooves by a plate or counter-plate hollowed out with grooves that are complementary to the grooves of the motherboard. This plate or counter-plate covers the layer of fibers positioned in the grooves of the motherboard so that each fiber is sandwiched in the hollow formed by two grooves and is held in position.
The problem then is to transfer the integrated optic circuit chip with precision on to the motherboard, in making the waveguides coincide with the optic fibers.
An article by W. HUNZIKER et al., entitled "Self-aligned Flip-chip Packaging of Tilted Semiconductor Optical Amplifier Arrays on Si Motherboard" in "Electronics Letters", March 1995, Vol. 31, No. 6, describes a technique for transferring a chip comprising waveguides to a motherboard hollowed out with grooves, with automatic positioning, the guides being embedded in the grooves. The fixing of the chip to the motherboard is obtained by the melting of intercalary solder stripes.
One drawback of this technique is that the guides undergo mechanical constraints that are harmful to the transmissions of optical signals.
Another drawback of this technique is that it is appropriate only when the waveguides have dimensions in the range of those of the grooves and the fibers, which is generally not the case since the dimensions of the waveguides varying according to the type of substrate.
An article by Q. LAI and the same authors, entitled "Silica on Si waveguides for self-aligned fibre array coupling using flip-chip Si V-groove Technique", in September 1996, in "Electronics Letters", Vol. 32, No. 20, describes an improvement of this technique in which each guide is bordered by two raised lateral ribs. Each pair of ribs bordering a guide rests on the two flanks of a respective groove, thus positioning the guide in the axis of the groove and of the fiber.
One drawback of these techniques is that there is no provision for attaching the chip and the motherboard which are simply laid one on the other. One of the directional parameters of alignment therefore is not set with precision.
The attachment requires subsequent steps of glueing or soldering which have the drawback either of being irreversible or of taking the substrate to high temperature.
Another drawback of these techniques is that they provide for the formation of raised substrate ribs making it necessary to clean almost the entire surface of the chip except for the waveguides, which have to be etched at the same time, when the chip is being manufactured.
More generally, the drawback of these flip-chip techniques is that they make use of etching by anisotropic chemical attack of the substrate of the chip to form the ribs. This attack is specific to certain crystalline substrates and, in particular, is appropriate neither to glass nor to lithium niobate.
The object of the present invention is to design a bonding of miniature optical components enabling a positioning with micrometrical precision and a perfect alignment of the waveguides of the integrated optic circuit chip and of the fibers immobilized on the motherboard, without any of the above-mentioned drawbacks.
A goal of the invention is to provide for a bonding that gives reliable, stable and permanent attachment.
Another goal of the invention is to design a universal mounting method applicable to every kind of substrate, including glass, lithium niobate and polymers, capable of adapting to every topography of integrated optic circuit chip, whatever the arrangement of the waveguides, whether raised or not.
Finally, a goal of the invention is to make miniature optical components by passive assembly that is simple, capable of automation and costs little.